The present invention relates generally to integrated circuits, and more particularly to voltage level converting circuits for shifting digital signals between two different voltage levels.
With recent technological developments, integrated circuits (IC) with very low operating voltages are being used more often. In a circuit that has two ICs operating at different internal operating voltages, a voltage level converter is used to facilitate the transfer of signals between the two ICs. For example, if the first IC, operating at 1.2V, is used in conjunction with the second IC, operating at 2.5V, a voltage level converter is incorporated in the second IC. The voltage level converter translates the output signal of the first IC from the operating voltage level of 1.2V to an input voltage of 2.5V for the second IC.
Voltage level converter circuits are typically made using a plurality of Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET). A MOSFET is characterized by the thickness of its gate oxide layer. The thickness of the gate size affects the switching speed and voltage range over which the transistor can operate. By reducing the thickness of the gate oxide layer, it is possible to increase the switching speed of the transistor.
The thickness of the gate oxide affects the voltage range over which the transistor is operable because a transistor can operate only if the voltage across its gate and its source is greater than a threshold voltage. The threshold voltage depends on the gate size. Hence, it is essential that the gate size be suitably selected. This gate size selection is of particular importance in the case of a voltage level converter. For example, the voltage level converter receives an input signal at the first voltage level and generates an output signal at the second voltage level. Hence, the gate size of each of the transistors should be selected so that each transistor can operate appropriately.
Conventional voltage level converters use transistors having a thick layer of oxide. These transistors are referred to as thick oxide transistors. Some voltage level converters use a combination of thick and thin oxide transistors.
Thick oxide transistors cannot withstand very low operating voltages. For example, thick oxide transistors have a threshold voltage of around 0.7V and an operating voltage of around 1V. Since the operating and threshold voltages are close, the reliability and performance of the voltage level converter is adversely affected. Consequently, voltage level converters with thick oxide transistors are not efficient or reliable at low voltages and limit the frequency of operation as well. Also, manufacturing thick oxide transistors requires extra masks and hence, more steps are required in the fabrication process, which leads to an increase in the cost of manufacture.
Therefore, there is a need for cost effective voltage level converters that can operate reliably with ICs, which have low operating voltages.